36.4 Creating an Asynchronous Process

In this section, we describe how to create an asynchronous
process. After an asynchronous process is created, it runs in
parallel with Emacs, and Emacs can communicate with it using the
functions described in the following sections (see Input to Processes, and see Output from Processes). Note that process
communication is only partially asynchronous: Emacs sends data to the
process only when certain functions are called, and Emacs accepts data
from the process only while waiting for input or for a time delay.

An asynchronous process is controlled either via a pty
(pseudo-terminal) or a pipe. The choice of pty or pipe is made
when creating the process, based on the value of the variable
process-connection-type (see below). Ptys are usually
preferable for processes visible to the user, as in Shell mode,
because they allow for job control (C-c, C-z, etc.)
between the process and its children, whereas pipes do not. For
subprocesses used for internal purposes by programs, it is often
better to use a pipe, because they are more efficient, and because
they are immune to stray character injections that ptys introduce for
large (around 500 byte) messages. Also, the total number of ptys is
limited on many systems and it is good not to waste them.

Function: start-processname buffer-or-name program &rest args

This function creates a new asynchronous subprocess and starts the
program program running in it. It returns a process object that
stands for the new subprocess in Lisp. The argument name
specifies the name for the process object; if a process with this name
already exists, then name is modified (by appending ‘<1>’,
etc.) to be unique. The buffer buffer-or-name is the buffer to
associate with the process.

If program is nil, Emacs opens a new pseudoterminal (pty)
and associates its input and output with buffer-or-name, without
creating a subprocess. In that case, the remaining arguments
args are ignored.

The remaining arguments, args, are strings that specify command
line arguments for the subprocess.

In the example below, the first process is started and runs (rather,
sleeps) for 100 seconds (the output buffer ‘foo’ is created
immediately). Meanwhile, the second process is started, and
given the name ‘my-process<1>’ for the sake of uniqueness. It
inserts the directory listing at the end of the buffer ‘foo’,
before the first process finishes. Then it finishes, and a message to
that effect is inserted in the buffer. Much later, the first process
finishes, and another message is inserted in the buffer for it.

Like start-process, this function starts a new asynchronous
subprocess running program in it, and returns its process
object.

The difference from start-process is that this function may
invoked a file handler based on the value of default-directory.
This handler ought to run program, perhaps on the local host,
perhaps on a remote host that corresponds to default-directory.
In the latter case, the local part of default-directory becomes
the working directory of the process.

This function does not try to invoke file name handlers for
program or for the program-args.

Depending on the implementation of the file handler, it might not be
possible to apply process-filter or process-sentinel to
the resulting process object. See Filter Functions, and Sentinels.

Some file handlers may not support start-file-process (for
example the function ange-ftp-hook-function). In such cases,
this function does nothing and returns nil.

Function: start-process-shell-commandname buffer-or-name command

This function is like start-process, except that it uses a shell
to execute the specified command. The argument command is a shell
command name. The variable shell-file-name specifies which shell to
use.

The point of running a program through the shell, rather than directly
with start-process, is so that you can employ shell features such
as wildcards in the arguments. It follows that if you include any
arbitrary user-specified arguments in the command, you should quote them
with shell-quote-argument first, so that any special shell
characters do not have their special shell meanings. See Shell Arguments. Of course, when executing commands based on user input
you should also consider the security implications.

Function: start-file-process-shell-commandname buffer-or-name command

This function is like start-process-shell-command, but uses
start-file-process internally. Because of this, command
can also be executed on remote hosts, depending on default-directory.

Variable: process-connection-type

This variable controls the type of device used to communicate with
asynchronous subprocesses. If it is non-nil, then ptys are
used, when available. Otherwise, pipes are used.

The value of process-connection-type takes effect when
start-process is called. So you can specify how to communicate
with one subprocess by binding the variable around the call to
start-process.

(let ((process-connection-type nil)) ; use a pipe
(start-process …))

To determine whether a given subprocess actually got a pipe or a pty,
use the function process-tty-name (see Process Information).